I *suspect* (just a guess) that the hippocampal neurons play a relatively small role in developing intelligence in the big picture. I think dendrite growth and snaptic connectivity and various other factors yet to be better understood will turn out to be more important to intelligence.

From my perspective, the discovery of new neurons was incredibly surprising, almost a game changer in theory, regarding the genetics of neuron apoptosis so I think we tend to lay a lot on that discovery on that basis alone rather than because they are actually especially critical to problem solving. There's a lot more to problem solving than memory and a relatively tiny number of neurons that exhibit this remarkable behavior.

I think this behavior will turn out to be more critical if we end up being able to trigger it in other parts of the brain as well, selectively. But that's risky business: pruning is sometimes as important to learning as new connections.